Control, monitoring and powering of a subsea hydrocarbon fluid production well for example is effected from a surface or land-based platform via an umbilical cable which carries the necessary electric/hydraulic power lines and electrical/optical communication lines. This umbilical is connected at its lower end to, typically, a subsea control and distribution unit (CDU), with the control signals/power being distributed to the various trees at the well heads on the sea bed as required. In the event of a failure of the umbilical a Back-Up Intervention Control (BUIC) system may be employed to replace the services normally provided by the umbilical in order to continue fluid production from the well until the umbilical is repaired. A BUIC system is effectively an insurance policy against failure of the prime controls umbilical, and is typically purchased where the prime umbilical is considered sufficiently vulnerable. A BUIC system is designed to be deployed only when a failure in the prime umbilical occurs, and a vessel is used for this deployment. Since the cost of maintaining a vessel to provide this service is prohibitive, most well operators opt for a BUIC system that employs a “vessel of opportunity” with limited facilities. While there-is generally assistance from a Remote Operated Vehicle (ROV) to enable disconnection of the umbilical and connection of the back-up umbilical at the CDU, there is generally no lift assistance available.
A typical deployment of a BUIC system is shown in FIG. 1. A vessel 1, for example a vessel of opportunity, carries a replacement dynamic umbilical 2 stowed on a reel/winch assembly 3. The assembly 3 is integrated with a handling/overboarding mechanism used to deploy the umbilical 2. Integrated into this system is an Emergency Quick Disconnect Package (EQDP) 4 which permits simple disconnection of the umbilical 2. Furthermore, the umbilical 2 may include buoyancy devices 5 to support the umbilical within the water. The umbilical 2, and umbilical-mounted half of the EQDP 4 are typically stowed on the reel 3 for handling through the handling/overboarding mechanism. The buoyancy devices 5 may also be stowed on the reel 3 if the operator requests this facility, alternatively they may be attached to the umbilical on installation. The lower end of the umbilical is connected to a subsea umbilical termination (SUT) 6. This provides connection with a CDU 8 via CDU receptacle 7.
To deploy the umbilical 2, it is wound from the reel 3, through the handling/overboard mechanism and over the side of the vessel 1. The umbilical 2 is required to support its own weight, plus that of the SUT 6, through the water column under the prevailing weather, sea and current conditions. For connection with the well installation, the umbilical 2 and its termination 6 must be presented directly above and correctly oriented to the receptacle 7. This is often possible in light weather and sea conditions, but is unlikely to be successful in any other sea state, and is a difficult and thus expensive exercise. The vessel of opportunity is unlikely to be as stable a platform as a larger installation vessel, and will therefore have greater pitch, heave and roll motions for any given set of weather or sea conditions. Such a vessel does not generally have a moonpool facility, which means that overboarding of the umbilical, in the worst case scenario, has to be over the stem of the vessel. This is probably the worst location at which to overboard a dynamic umbilical when trying to position the SUT at installation.